Re: [PATCH 2/2] Revert "drm/i915/dp: change aux_ctl reg read to polling read"

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 19 Nov 2025, Ville Syrjala <ville.syrjala@linux.intel.com> wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>
> This reverts commit 5a9b0c7418448ed3766f61ba0a71d08f259c3181.
>
> The switch from AUX interrupts to pollign was very hand-wavy.
> Yes, there have been some situations in CI on a few platforms
> where the AUX hardware seemingly forgets to signal the timeout,
> but those have been happening after we switched to polling as
> well. So I don't think we have any conclusive evidence that
> polling actually helps here.
>
> Someone really should root cause the actual problem, and see
> if there is a proper workaround we could implemnt (eg. disabling
> clock gating/etc.). In the meantime just go back to using the
> interrupt for AUX completion.
>
> If the hardware fails to signal the timeout we will just hit
> the wait_event_timeout() software timeout instead. I suppose
> we could try to tune the software timeout to more closely
> match the expected hardware timeout. Might need to use
> wait_event_hrtimeout() or something to avoid jiffies
> granularity issues...
>
> The AUX polling is also a hinderance towards using poll_timeout_us()
> because we have a very long timeout, but would need a fairly short
> polling interval to keep AUX transfer reasonably fast. Someone would
> need to come up with good numbers in a somewhat scientific way.
>
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

Acked-by: Jani Nikula <jani.nikula@intel.com>

> ---
>  drivers/gpu/drm/i915/display/intel_dp_aux.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux.c b/drivers/gpu/drm/i915/display/intel_dp_aux.c
> index 809799f63e32..d1a93e4a59b5 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_aux.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_aux.c
> @@ -6,6 +6,7 @@
>  #include <drm/drm_print.h>
>  
>  #include "intel_de.h"
> +#include "intel_display_jiffies.h"
>  #include "intel_display_types.h"
>  #include "intel_display_utils.h"
>  #include "intel_dp.h"
> @@ -60,16 +61,17 @@ intel_dp_aux_wait_done(struct intel_dp *intel_dp)
>  	i915_reg_t ch_ctl = intel_dp->aux_ch_ctl_reg(intel_dp);
>  	const unsigned int timeout_ms = 10;
>  	u32 status;
> -	int ret;
> +	bool done;
>  
> -	ret = intel_de_wait_ms(display, ch_ctl,
> -			       DP_AUX_CH_CTL_SEND_BUSY, 0,
> -			       timeout_ms, &status);
> +#define C (((status = intel_de_read_notrace(display, ch_ctl)) & DP_AUX_CH_CTL_SEND_BUSY) == 0)
> +	done = wait_event_timeout(display->gmbus.wait_queue, C,
> +				  msecs_to_jiffies_timeout(timeout_ms));
>  
> -	if (ret == -ETIMEDOUT)
> +	if (!done)
>  		drm_err(display->drm,
>  			"%s: did not complete or timeout within %ums (status 0x%08x)\n",
>  			intel_dp->aux.name, timeout_ms, status);
> +#undef C
>  
>  	return status;
>  }

-- 
Jani Nikula, Intel